1. Field of the Invention
This invention is in the field of laser radar devices and, more specifically, to systems and methods remotely detecting the presence of species distributed in the atmosphere, such as biological species.
2. Relevant Background
Light Detection and Ranging (lidar) systems are used in numerous areas of practical interest to make remote measurements. In lidar systems a light beam is sent to a target and a detection system is used to extract information about the target. In some cases the target is classified as a “hard-target” and may consist of a material having properties or a location that one desires to determine. In other cases targets are distributed, meaning generally that they are not localized in space. Examples of distributed targets include particles suspended in the atmosphere, or aerosols, such as water droplets, sea salt, spores, pollen and dust; chemical plumes such as pipeline natural gas leaks and paint fumes; and background atmospheric gases such as ozone, water vapor and carbon dioxide.
Heightened concerns in recent years about potential use of airborne chemical and biological agents to cause harm has increased the urgency of finding methods to remotely detect and locate such agents. It has generally been found difficult to use remote sensing methods to a) detect threat species and b) reliably discriminate the threat species from other species that may also be present. In the case of biological agents like anthrax it is in principle possible to detect their presence by simply collecting scattered light from the particles. However, simple light scattering measurements often cannot tell the difference between types of aerosols, so the scattered light “signature” of an anthrax particle is similar for example, to the scattered light signature of common dust.
To enhance stand-off biological agent discrimination, other laser based remote sensing techniques have been developed, in particular Laser Induced Fluorescence or LIF. In LIF a short wavelength (typically in the UV or visible spectral range) laser illuminates the particles, the light is absorbed and subsequently re-emitted at a different (longer) wavelength. By detecting the longer wavelength emission one may infer that a biological aerosol is present (since inorganic materials tend not to fluoresce). However, there is frequently little in the fluorescence signature that permits one to distinguish one biological species from another.
Other techniques such as Raman lidar have been applied to chemical concentration mapping and aerosol extinction measurements but are not useful for aerosol discrimination or identification. Differential absorption lidar (DIAL) is another method that permits one to probe spectral absorbing features remotely by tuning a probe laser on and off the absorption line. However, biological species are chemically similar to each other and have spectrally broad absorption features similar to chemicals such as hydrocarbons. This makes it very difficult to spectrally distinguish hazardous bioaerosol from benign bioaerosol and other background atmospheric chemicals. Aerosol depolarization measurements have also been investigated and used to distinguish stratospheric ice from water droplets. However, application of this technique to bioaerosol discrimination has proven to be only weakly effective with depolarization variations due to the type of aerosol being much less pronounced than changes due to the aerosol concentration (see, for example, J. H. Marquardt et al., “Measurement of bio-aerosols with a polarization-sensitive, coherent Doppler lidar”, 5th Joint Conference on Standoff Detection for Chemical and Biological Defense, Williamsburg, Va., Sep. 24-28, 2001). This prevents pure depolarization measurements from serving as an aerosol discriminator outside of laboratory conditions. Passive methods (see e.g. Theriault et al. “Passive standoff detection of BG aerosol. Method and field trial results”, Proc. of SPIE pp. 163, vol. 5268, 2004) have also been used but do not lend themselves to a high degree of discrimination between species especially at stand-off ranges.
What is needed is a method that is capable of and discriminating hazardous biological agents to enable suitable action to be taken when a threat species is found. In addition the method should desirably permit detection at stand-off ranges of hundred of meters or even more desirably several kilometers. The technique should also be capable of day or night operation.